Modern vehicle powertrains employ a hydraulic torque converter to couple engine torque to an automatic transmission. As is known in the art, the torque converter has an input member driven by the engine, and an output member coupled to the input of the transmission. The torque converter's input and output members each have a turbine, and the turbines are immersed in, and rotationally coupled through, a hydraulic fluid. The hydraulic fluid and turbines do not, however, provide a 100% coupling of rotational speed between the input and output members. An electronically modulated converter clutch (EMCC) may therefore be used in addition to the turbines to selectively and gradually increase coupling between the input and output members. Such an arrangement is disclosed in commonly-owned U.S. Pat. No. 4,993,527.
Completely coupling the input and output members with an EMCC provides a fuel economy benefit, however complete coupling may also cause undesirable noise, vibration, and harshness (NVH) responses in the vehicle driveline. The primary cause of these NVH characteristics is torsional vibration from cylinder firings in the engine. It is therefore desirable to allow a limited amount of EMCC slip to dampen the vibrations. EMCC slip is the RPM difference between the torque converter's input and output members. But because the coefficient of friction between the input and output members of the EMCC varies with several independent variables, such as component wear and temperature, the slip is often oscillatory in nature and introduces NVH issues of its own. Therefore, allowing limited slip during steady state vehicle operation has been an elusive goal.